Method and apparatus for characterization and compensation of optical impairments in InP-based optical transmitter

ABSTRACT

A method and apparatus for characterizing and compensating optical impairments in an optical transmitter includes operating an optical transmitter comprising a first and second parent MZ, each comprising a plurality of child MZ modulators that are biased at respective initial operating points. An electro-optic RF transfer function is generated for each of the plurality of child MZ modulators. Curve fitting parameters are determined for each of the plurality of electro-optic RF transfer functions and operating points of each child MZ modulator are determined using the curve fitting parameters. An IQ power imbalance is determined using the curve fitting parameters. Initial RF drive power levels are determined that compensate for the determined IQ power imbalance. The XY power imbalance is determined for initial RF drive power levels using the curve fitting parameters. The operating RF drive powers are determined that at least partially compensate for the first and second IQ power imbalances and for the XY power imbalance for the optical transmitter. An optical signal comprising a Nyquist-pulse-shape is generated at an output of the optical transmitter.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 15/488,507, filed on Apr. 16, 2017, entitled“Method and Apparatus for Characterization and Compensation of OpticalImpairments in InP-Based Optical Transmitter”, which is a continuationof U.S. patent application Ser. No. 14/975,738, filed on Dec. 19, 2015,entitled “Method and Apparatus for Characterization and Compensation ofOptical Impairments in InP-Based Optical Transmitter”. The entirecontents of U.S. patent application Ser. Nos. 15/488,507 and 14/975,738are herein incorporated by reference.

The section headings used herein are for organizational purposes onlyand should not to be construed as limiting the subject matter describedin the present application in any way.

INTRODUCTION

The ever increasing demand for optical fiber communication capacitycontinues to drive improvements in optical transmitter technology usedin long-haul and metro optical network deployments. The need to addressthe combination of large bandwidth requirements, high port density, andlower system power consumption continue to push technology limits.In-Phase Quadrature-Phase (IQ) optical modulators support the highmodulation bandwidths in today's coherent optical systems. Importantmodulator performance parameters for this application include low drivevoltage to produce a phase shift of π-radians, V_(π), high linearity,high modulation bandwidth, and low insertion loss. In addition,high-capacity systems demand a small form factor and high componentreliability.

Current generation IQ modulators rely heavily on lithium niobate LiNbO₃technology. However, LiNbO₃ modulators have fundamental limitations onthe modulator size needed to achieve low drive voltages. Compound III-Vsemiconductor-based modulator technologies have the potential for highbandwidth and compact device configurations, and III-V devices arealready widely used as optical laser sources and detectors in currentlydeployed telecommunications systems. Indium phosphide (InP) technology,in particular, is well suited for modulating telecommunication systemwavelengths. Indium phosphide technology is also compatible withwafer-scale fabrication that allows precise process controls and can beused with low cost packaging. These features of indium phosphidetechnology have dramatically reduced the cost of InP modulatorcomponents, enabling InP modulators to have an acceptable cost pertransmitted bit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teaching, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed description, taken inconjunction with the accompanying drawings. The skilled person in theart will understand that the drawings, described below, are forillustration purposes only. The drawings are not necessarily to scale,emphasis instead generally being placed upon illustrating principles ofthe teaching. The drawings are not intended to limit the scope of theApplicant's teaching in any way.

FIG. 1 illustrates an embodiment of an optical transmitter impairmentcharacterizer and compensator according to the present teaching.

FIG. 2 illustrates an embodiment of a modulator drive amplifieraccording to the present teaching.

FIG. 3 illustrates a method for characterization and compensation ofoptical impairments in optical transmitters according to the presentteaching.

FIG. 4 illustrates a plot used to determine MZ modulator RF drivingvoltage parameters using the method of the present teaching.

FIG. 5 illustrates a plot of the measured electro-optic RF transferfunctions for four MZ modulators embedded into a wavelength tunableInP-based optical transmitter.

FIG. 6A illustrates the transmitter modulator power imbalances as afunction of wavelength before compensation.

FIG. 6B illustrates the transmitter modulator power imbalances as afunction of wavelength after compensation using the method of thepresent teaching.

FIG. 7A illustrates measured performance data from a DP-QPSK transmitterwithout compensation.

FIG. 7B illustrates a continuation of the measured performance data fromthe DP-QPSK transmitter without compensation shown in FIG. 7A.

FIG. 8A illustrates measured performance data from a DP-QPSK transmitterusing an embodiment of the compensation method of the present teaching.

FIG. 8B illustrates a continuation of measured performance data from aDP-QPSK transmitter using an embodiment of the compensation method ofthe present teaching.

FIG. 9 illustrates a block diagram of an embodiment of an opticaltransmitter of the present teaching that includes a transmit DSPcomprising high-speed digital-to-analog converters.

FIG. 10A illustrates a graph of the peak-to-average power ratio (PAPR)for pulse amplitude modulation with four levels (PAM4) as a function ofthe Nyquist pulse roll-off factor.

FIG. 10B illustrates a graph of the optical power for sixteen-levelquadrature amplitude modulation (16QAM) as a function of the Nyquistpulse roll-off factor.

FIG. 11A illustrates a graph of the peak-to-average power ratios fornQAM signals generated by an embodiment of an optical transmitter of thepresent teaching.

FIG. 11B illustrates a graph of the peak-to-average power ratio fordifferent pulse roll-off factors for raised cosine filter transferfunction with 50% Tx/Rx split Nyquist pulse shaping generated by anembodiment of an optical transmitter according to the present teaching.

DESCRIPTION OF VARIOUS EMBODIMENTS

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the teaching. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

It should be understood that the individual steps of the methods of thepresent teachings may be performed in any order and/or simultaneously aslong as the teaching remains operable. Furthermore, it should beunderstood that the apparatus and methods of the present teachings caninclude any number or all of the described embodiments as long as theteaching remains operable.

The present teaching will now be described in more detail with referenceto exemplary embodiments thereof as shown in the accompanying drawings.While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. Those of ordinary skill inthe art having access to the teaching herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein.

Recent developments have shown the ability of InP-based opticaltransmitter modulators to provide relatively high-bandwidth, relativelylow drive voltage, and relatively low insertion loss. However, InP-basedmodulators are still not suitable for widespread deployment for variousreasons. In particular, improvements need to be made in minimizingwavelength dependence of IQ power imbalance and XY power imbalance indual-polarization in-phase and quadrature-phase InP optical modulatorsto generate ideal signal constellations for transmission in long-hauland metro optical networks.

One feature of the present teaching is to provide an apparatus and amethod that first characterizes and then compensates for wavelengthdependence of IQ power imbalance and XY power imbalance of adual-polarization in-phase and quadrature optical transmitter modulatorbased on InP technology. An apparatus according to the present teachingincludes a characterizer that actively determines parameters related tothe IQ and XY power imbalances of the transmitter, and also acompensator that at least reduces and, in some cases, removes the IQ andXY power imbalances in the transmitter at any or all wavelengths.

FIG. 1 illustrates an embodiment of an optical transmitter modulatorimpairment characterizer and compensator 100 according to the presentteaching. The optical transmitter impairment characterizer andcompensator 100 include a transmitter optical subassembly (TOSA) 102. Intelecommunication and data communication networks and links, a TOSA issubassembly that is part of the optical transceiver system for sendingand receiving data across an optical fiber. The TOSA 102 converts anelectrical signal into an optical signal that is coupled into an opticalfiber. In some embodiments, the transmitter optical subassembly 102 ishoused in a gold enclosure. As the TOSA 102 is designed to interoperatewith other optical subassembly units, TOSA 102 operating performancemust be established and controlled using simple and effectivecharacterization and compensation systems.

In some embodiments, the transmitter optical sub assembly 102 is adual-polarization in-phase (I) and quadrature (Q) optical transmittermodulator. The dual-polarization in-phase (I) and quadrature (Q) opticaltransmitter modulator includes four child Mach-Zehnder (MZ) modulators104, 106, 108, 110 nested to form a pair of parent MZ modulatorsreferred to herein as a first 112 and second 114 parent MZ modulator. Insome embodiments, the dual-polarization in-phase (I) and quadrature (Q)optical transmitter modulator uses InP-based optical modulatorcomponents. An optical input 116 of the dual-polarization in-phase (I)and quadrature (Q) optical modulator is optically coupled to the opticalinputs of the first 112 and second 114 parent MZ modulator. The acronymMZM as used herein refers to Mach-Zehnder modulator.

The transmitter optical sub assembly 102 also includes a tunable lasersource 118 that in some embodiments is a full C-band-wavelengththermally-tunable laser source. In the embodiment shown in FIG. 1, anoutput of the tunable laser 118 is optically coupled to the opticalinput 116 of the dual-polarization in-phase (I) and quadrature (Q)optical modulator. The transmitter optical sub assembly 102 produceswavelength tunable modulated optical signals, including 100-Gb/s DP-QPSKand/or 200-Gb/s DP-16QAM format optical signals, at an optical output120.

In the configuration shown in FIG. 1, each of the first 112 and second114 parent MZ modulator modulates one of the two polarizations in thedual-polarization modulator. For example, in the embodiment illustratedin FIG. 1, the MZ modulators 104, 106 are child MZ modulators formingthe parent MZ modulator 112. The MZ modulator 106 is optically connectedto a π/2 phase shifter 122. The output of the phase shifter 122 and theoutput of MZ modulator 104 are combined to form the output of parent MZmodulator 112 that generates an X-polarized modulated optical beam.Parent MZ modulator 112 is also referred to herein as the X-Pol.modulator. Similarly, the MZ modulators 108, 110 are child MZ modulatorsassociated with the parent MZ modulator 114. The MZ modulator 110 isoptically connected to a π/2 phase shifter 124. The output of the phaseshifter 124 and the output of MZ modulator 108 are combined and passthrough a 45° aligned polarization rotator 126 to form the output ofparent MZ modulator 114 that generates the Y-polarized modulated opticalbeam. Parent MZ modulator 114 is also referred to herein as the Y-Pol.modulator.

In operation of the dual-polarization optical transmitter, the MZmodulators 104, 106 modulate in-phase and quadrature phase,respectively, on the X polarization optical beam modulated by the parentMZ modulator 112. The MZ modulators 108, 110 modulate in-phase andquadrature-phase, respectively, on an optical beam that passes through a45° aligned polarization rotator 126 to provide Y polarization lightmodulated by the parent MZ modulator 114. The output of parent MZmodulator 114 and the output of parent MZ modulator 112 are combined toform the output 120 of the dual-polarization in-phase (I) and quadrature(Q) optical modulator of the transmitter optical subassembly 102.

The electrical modulation inputs of the MZ modulators 104, 106, 108, 110are each connected to the output of a respective modulator driveamplifier 128, 130, 132, and 134. The modulator drive amplifiers 128,130, 132, and 134 each supply modulation signals to the respective MZmodulators. In some embodiments, the modulator drive amplifiers 128,130, 132, and 134 are linear radio-frequency modulator drive amplifiers.The modulator drive amplifier 128 connected to MZM-XI 104 has a positivedifferential input port XIP 136, and a negative differential input portXIN 138. The modulator drive amplifier 130 connected to MZM-XQ 106 has apositive differential input port XQP 140, and a negative differentialinput port XQN 142. The modulator drive amplifier 132 connected toMZM-YI 108 has a positive differential input port YIP 144, and anegative differential input port YIN 146. The modulator drive amplifier134 connected to MZM-YQ 110 has a positive differential input port YQP148, and a negative differential input port YQN 150.

The optical transmitter impairment characterizer and compensator 100also include a signal generator 152. For example, the signal generator152 may be a 1-GHz RF sinusoidal signal generator that produces varyingRF output power levels. One feature of the present teaching is the useof relatively low RF frequencies for characterization, even as the datarates of the optical transmitter are relatively high. Using a signalgenerator with a low, 1-GHz bandwidth advantageously allows thecharacterization and compensation system to be implemented with alow-cost signal generator. The 1 GHz RF frequency has been shown to besufficient to characterize the electro-optic response curve of the MZmodulators for operation of the optical transmitter at rates of 100 Gb/sor 200 Gb/s. In various embodiments, the signal generator 152 operatesin an RF frequency range of 500 MHz to 3 GHz. This range of frequenciesprovides an acceptable characterization of the electro-optic RF responseof the various MZ modulators, and also allows the embodiments to uselow-cost signal generators. In one specific embodiment, the RF outputpower level of a 1-GHz sinusoidal signal source is varied between −15dBm and +6 dBm.

The single-ended output of the signal generator 152 is electricallyconnected to a 180-degree RF hybrid 154. In some embodiments, the180-degree RF hybrid 154 is a 3 GHz low-cost hybrid. The 180-degree RFhybrid 154 converts the single-ended output of the signal generator 152to a differential output having a positive output 156 and negativeoutput 158. The positive differential output 156 of the 180-degree RFhybrid 154 is electrically connected to a first 4-position RF Switch160. The first 4-position RF Switch 160 switches the input amongstoutputs at four ports including a first port 162 that is electricallyconnected to differential input port XIP 136, a second port 164 that iselectrically connected to differential input port XQP 140, a third port166 that is electrically connected to differential input port YIP 144, afourth port 168 that is electrically connected to differential inputport YQP 146.

The negative differential output 158 of the 180-degree RF hybrid 154 iselectrically connected to a second 4-position RF Switch 170 thatswitches the input amongst outputs at four ports, a first port 172 thatconnects to differential input port XIN 138, a second port 174 thatconnects to differential input port XQN 142, a third port 176 thatconnects to differential input port YIN 146, a fourth port 178 thatconnects to differential input port YQN 150. The connections between thesignal generator 152, 180-degree RF hybrid 154, the first and second4-position RF Switches 160, 170, modulator drive amplifiers 128, 130,132, and 134 and MZ modulators 104, 106, 108, 110 are electrical RFconnections. Thus, the two RF switches 160, 170 serve to direct thedifferential output of the 180-degree RF hybrid 154 to the differentialinputs of the RF modulator driver amplifiers 128, 130, 132, 134.

The characterizer and compensator 100 also include an optical powermeter 180 having an optical input coupled to the optical output 120 ofthe transmitter optical sub assembly 102. In some embodiments, theoptical power meter 180 is a wavelength calibrated optical power meterwith a detector bandwidth of greater than 1 GHz. The optical power meter180 detects RF modulation initiated at the signal generator 152 andimposed by the MZ modulators 104, 106, 108, 110 onto the wavelengthtunable optical signal generated by the tunable laser source 118. The RFswitches 160, 170 are used to cycle the RF modulation signals used forcharacterization onto each MZ modulator 104, 106, 108, 110 individually,so that the response of each MZ modulator 104, 106, 108, 110 may beindividually characterized.

A processor 182 is used to collect data measured by the characterizer,and also to provide control outputs for the compensation by thecharacterizer and compensator 100. For characterization, the processor182 is connected to an electrical output of the optical power meter 180and an electrical output of the signal generator 152 using a controlpath. In some embodiments, the control path is provided by a generalpurpose interface bus (GPIB). The processor 182 collects the opticalpower data from the optical power meter 180 as a function of the signalgenerator power level provided by the signal generator 152 at variouswavelengths output by tunable laser 118. The processor 182 may alsoderive RF output power information from the modulator drive amplifiers128, 130, 132, 134. The RF output power of the driver amplifier is theRF input drive power to the MZM. The RF input power to the MZM is alsocommonly referred to as RF drive power or RF input drive power. Thus,the processor may collect the data representing the RF input power tothe MZ modulators directly from the modulator drive amplifiers using acontrol path connection.

In some embodiments, the signal generator 152 generates acharacterization RF modulation signal by ramping the power level of anRF tone or frequency from a low power to a high power. The particularstarting and ending power levels of the characterization RF modulationsignal depends on the V_(π) of the MZ modulators 104, 106, 108, 110. Themetric V_(π) is the peak-to-peak input RF voltage to the MZ modulatorrequired to induce a phase shift of π radians onto the optical signal ofthe MZ modulator being driven. In many embodiments, the RF power of thesignal generator 152 is varied between −15 dBm and +6 dBm depending onthe V_(π) of the MZ modulator.

In operation, the processor 182 determines the wavelength dependentin-phase and quadrature power imbalance by processing the data of themeasured optical power as a function of signal generator drive power atvarious wavelengths. More specifically, the processor 182 determines theIQ-power imbalance as a power difference between I-child and Q-child MZmodulator's output optical powers per polarization of thedual-polarization in-phase and quadrature optical modulator. Theprocessor 182 also determines the polarization power imbalance byprocessing the power difference between the modulated output power ofthe parent MZ modulator 112 and of the parent MZ modulator 114. That is,the processor 182 determines XY-power imbalance as a power differencebetween the light modulated in the X-polarization and theY-polarization.

The processor 182 is electrically connected to control inputs associatedwith modulator drive amplifiers 128, 130, 132, and 134 associated withrespective MZ modulators 104, 106, 108, 110. The processor 182 sendscontrol signals to the drive amplifiers 128, 130, 132, and 134 that areused to manually set the voltage set points of the drive amplifiers toachieve a specific gain. In some embodiments, the modulator driveamplifiers 128, 130, 132, and 134 are controlled to operate in anautomatic gain control mode of operation to unequally drive therespective RF modulation inputs of the MZ modulators 104, 106, 108, and110.

The characterizer and compensator 100 generates at an optical output 120signals that are compensated for optical impairments, such as wavelengthdependent IQ power imbalance and XY power imbalance, of the InP-basedtransmitter optical sub assembly 102. In one embodiment, the opticaloutput 120 of the compensator of the characterizer and compensator 100provides a 100-Gb/s DP/QPSK and/or 200-Gb/s DP-16QAM opticalcommunication signal that is compensated for wavelength dependent andpolarization dependent power balances, including IQ power imbalance andXY power imbalance.

One feature of the characterizer and compensator 100 of the presentteaching is that characterization and compensation can be performed inthe field. Internal photodetectors 184, 186, and 188 are connected tothe output of the two parent MZ modulators 112, 114, and also to thecombined output 120 of the transmitter optical sub assembly 102. Fieldcompensation can be used to compensate for component aging in the fieldthat causes IQ and XY power imbalances.

The IQ power imbalance is somewhat independent of temperature in partbecause the I and Q child MZ modulators are embedded into a parent MZmodulator super structure built onto a monolithic substrate. Each IQparent MZ modulator chip physically sits on a thermo-electric cooler(TEC) operated to maintain a particular temperature, which is set to anoperating temperature in range from 40° C. to 50° C. Moreover, RFautomatic gain control is running on RF driver amplifiers usingtemperature compensated RF peak detector diodes. Consequently, factorycalibration is commonly sufficient as far as IQ imbalance variation isconcerned.

The XY power imbalance may occur in the field in some practicalembodiments because external bulk optical components are used topolarization multiplex the two outputs of the two separate IQ MZmodulator chips which perform IQ modulation and the coupling efficiencyis sensitive to temperature in some circumstances. To overcome thisdependence of XY power imbalance, a photodiode 188 is placed at theoutput 120 of the transmitter optical subassembly 102 to couple andmeasure light from the output of the TOSA 102 in the field.

Another feature of the characterizer and compensator 100 of the presentteaching is that characterization and compensation can be used tocompensate for thermal effects. A simple DC approach can be used toclose the temperature sensitive feedback loop. First, the responsivitiesof the two internal photodiodes 184, 186 connected to each parent MZmodulator 112, 114 are calibrated with respect to the output opticalpowers belonging to each respective polarization, X-Pol. and Y-Pol. Thenthe two photodiode 184, 186 output currents are used by the processor182 in normal operation to estimate the final output power exiting thetransmitter optical subassembly 102 in each polarization. This estimatedpower is compared to a total measured optical output power using a finaltap monitor photodiode 188 to generate an error signal that can be usedto compensate XY power imbalance variation.

One skilled in the art will appreciate that while the impairmentcharacterizer and compensator 100 of FIG. 1 is illustrated and describedin connection with a DP-QPSK/DP-16QAM-type transmitter, it should beunderstood that the impairment characterizer and compensator of thepresent teaching can be used with numerous other optical transmitters.Furthermore, while some embodiments of the present teaching aredescribed in connection with InP-based optical modulators, it will beunderstood to those familiar with the state-of-the art in opticaltransmitter technology that the compensator of the present teaching canbe applied to numerous other types of optical modulators.

In some embodiments, the drive voltage and RF input power applied to theMZ modulators by the modulator drive amplifiers 128, 130, 132, and 134is determined using an RF power readout provided directly from themodulator drive amplifiers 128, 130, 132, and 134. FIG. 2 illustrates anembodiment of a modulator drive amplifier 200 of the present teaching.Those familiar with the state-of-the-art will appreciate that any numberof modulator drive amplifier designs may be used in the presentteaching. The particular modulator drive amplifier 200 described inconnection with FIG. 2 includes an electrical amplifier 202 comprisingone of many electrical amplifier designs known in the art. In someembodiments, the electrical amplifier 202 comprises a two-stage orthree-stage driver amplifier that includes a first differentialamplifier stage, followed by a second and a third single-ended amplifierstage. The modulator drive amplifier 200 has two differential inputs204, 206 for applying positive and negative RF signals. The modulatordrive amplifier 200 has a single-ended output 208 that provides RFoutput voltages to a MZ modulator input. In some embodiments, themodulator drive amplifier may also have a differential output. Adifferential output allows, for example, a differential drive for a dualpolarization modulator, such as a InP dual-polarization IQ modulatorthat is driven differentially. Some embodiments of the modulator driveamplifier 200 include a peak detector 210, which may be an RF peakdetector diode, connected from the output 208 of the amplifier 200 toground.

The output power and hence voltage at the output of the modulator driveamplifier's output 200 can be read as a rms output voltage of the RFpeak detector 210 through a readout port 212 of the modulator driveamplifier 200. The modulator drive amplifier 200 includes a firmwarecomponent 214 that is used to control the output power and gain of theelectrical amplifier 202. The firmware component 214 has an input 216that allows users to provide various inputs to control the modulatordrive amplifier 200. For example, users can input a gain control voltagethat controls the RF output power provided by the electrical amplifier202 at the output 208 and/or the gain of the electrical amplifier 202from the input to the output. Users can also control whether the readoutand/or control functions are provided in voltage domain or in powerdomain. In some embodiments, the modulator drive amplifier 200 is alinear RF driver amplifier that is a variable gain amplifier having again control input. Gain is varied by adjusting the voltage applied torespective gain control inputs.

One feature of the present teaching is that the accuracy of thedetermined value of input RF power can be improved by reading thevoltage and/or power directly applied to the MZ modulator at the output208 of the modulator drive amplifier 200 using the readout port 212 ofthe peak detector 210 voltage. The peak detector 210 may be an RF peakdetector diode. In some embodiments, the RF peak detector diodes'readouts are read using analog-to-digital converters that are connectedvia a serial communication interface bus, such as a serial peripheralinterface bus (SPI) or an Integrated Circuit Bus (I2C). Referring bothto FIG. 1 and FIG. 2, the interface bus connects the processor 182 tothe differential amplifiers 128, 130, 132, 134 via the readout port 212.In some embodiments the electrical amplifier 202 is a lineardifferential input and single-ended output modulator driver amplifierwith RF automatic gain control (RF AGC). In some embodiments, the peakdetector diode 210 is an RF peak detector diode having a frequencyresponse that is greater than 15 GHz that is used to measure the RFoutput power at the modulator driver amplifier's output 208.

In some embodiments of the method of the present teaching, the peakdetector 210 is calibrated by pre-characterizing its DC output voltagereadout as a function of peak-to-peak RF input voltage for three testcases. The calibration provides a calibrated voltage or power providedto the input of each MZ modulator as a function of the voltage or powervalue being read off the RF peak detector. In some methods according tothe present teaching, this calibration is done before the steps ofcharacterization and compensation of optical impairments are performed.

The calibration of the RF peak detector 210 begins by measuring theresponse of the RF peak detector 210 at three points. The first datapoint is with no RF voltage applied, in other words, a DC offsetvoltage. That is, the RF peak detector diodes' DC offset voltage ismeasured when no RF input voltage is applied to the modulator driveramplifier and the modulator driver amplifier is itself connected to arated power supply. This DC offset voltage is used for RF peak detectorDC readout calibration. In particular, during calibration, this DCoutput offset voltage is subtracted from the measured RF peak detectors'output DC voltage when an RF input voltage is applied to the modulatordriver amplifiers' input.

For the second data point, the RF peak detector diodes' DC outputvoltage is measured when modulator driver amplifier is producing aselected value that is proportional to V_(pp), where V_(pp) indicates apeak-to-peak voltage, at its output. For example, in one particularembodiment, the modulator driver amplifier produces a selected valueequal to about 2.4 V_(pp). This peak-to-peak voltage corresponds to a RFpower level of +11.6 dBm. In this embodiment, the modulator driveamplifier is set to the 2.4V_(pp) value by adjusting the voltage appliedto the modulator driver amplifiers' gain control input. In otherembodiments, a different value is used to set the modulator driveamplifier via an appropriate adjustment of the voltage applied to themodulator driver amplifiers' gain control input. The selected values arechosen based on, for example, particular types of RF driver technologyand/or particular types of modulator technology that are used in thetransmitter.

For the third data point, the RF peak detector diodes' DC output voltageis measured when modulator driver amplifier is producing a selectedvalue that is proportional to V_(pp) at its output that, in oneparticular embodiment, is equal to about 3.7 V_(pp). This peak-to-peakvoltage corresponds to a RF power level of +15.3 dBm. These values areobtained, once again, by readjusting the voltage applied to themodulator driver amplifiers' gain control input.

The next step in the calibration is to determine the RF power beingsupplied by the differential amplifier as a function of the gain controlvoltage input applied to the differential amplifier using the three testdata points established for the RF peak detector calibration. The RFpeak detector's DC output voltage readout follows Shockley's' idealdiode equation and thus is an exponential function of the peak-to-peakRF input voltage that is applied to it. As such, a natural logarithm,which is a logarithm of a number to base e where e is irrational andtranscendental constant equal to 2.718281828459, is used to convert theRF peak detectors' DC readout voltage that is being read into a readoutthat is linearly proportional to the RF power that is being measured bythe RF peak detector diode. Before taking the natural logarithm of theRF peak detectors' DC output voltage, its DC output offset voltage issubtracted from the measured RF peak detector's DC output voltageproduced in response to an applied RF input voltage to it.

Two calibration data points corresponding to two other measurementpoints are then used to fit a linear equation, an equation characterizedby a straight line in slope-intercept form. The computed slopes andy-intercepts are used to accurately determine the RF power level reachedat the output of the corresponding modulator driver amplifiers at aparticular value of the voltage applied to their corresponding gaincontrol inputs. The RF output power level of the linear RF modulatordriver amplifiers can be controlled precisely over a designed dynamicrange of 6 dB using its gain control voltage input. The RF automaticgain control function is implemented in the firmware component 214 usinga proportional integral control algorithm. The RF automatic gainmaintains the MZ modulator's RF drive power level at a constant value.In this way, the MZ modulator RF drive power level can be knownprecisely as a function of the applied gain control voltage and/or thevalue of the RF peak detector readout voltage levels can be knownprecisely as a function of the applied gain control voltage using thecalibration the method described herein.

In some methods according to the present teaching, gain control voltagesare adjusted on each of the RF driver amplifiers after calibration, butbefore beginning the characterization of the optical transmitter. In onespecific method, these adjustments are made so that the calibrated RFpeak detector reads 2V_(pp), corresponding to +10 dBm RF output power atthe output of the RF driver amplifiers for a −13 dBm differential-inputfrom the 180-degree RF hybrid. The input to the 180-degree hybrid fromthe 1-GHz signal generator is set to −10 dBm so that the RF amplifierhas fixed 20-dB gain at the start of the RF power sweeps.

FIG. 3 illustrates an embodiment of a method 300 for characterizationand compensation of optical impairments in optical transmittersaccording to the present teaching. One skilled in the art willappreciate that not all steps in the method 300 are used in allembodiments of the present teaching. Also, some aspects of the method300 are described in connection with an InP-based optical transmitter.However, it is understood that the method 300 can be used with numerousother types of optical transmitters. For example, in some embodiments,the method 300 characterizes and then compensates the wavelengthdependence of IQ power imbalance and XY power imbalance of adual-polarization in-phase and quadrature optical modulator based on InPtechnology. It is understood that the present teachings are not limitedto such embodiments.

Referring to FIGS. 1, 2 and 3, in the first step 302 of the method 300,optical output power and RF input drive power of each child MZ modulator104, 106, 108, and 110 is measured across a range of applied RF inputpower levels for all wavelengths across the optical C-band. The RF inputdrive power levels applied to the MZ modulators in the first step 302are referred to as characterizing RF input drive power levels. Thecharacterizing RF input drive power levels when applied as RF inputdrive power to various child MZ modulators allows an optical outputpower of the transmitter optical subassembly 102 to be measured as afunction of the characterizing RF input drive power levels applied toeach child MZ modulator. The first step 302 represents measuringwavelength dependent electro-optic RF transfer function of the each ofthe child MZ modulators 104, 106, 108, and 110 by measuring opticaloutput power of each of child MZ modulator and the respectivecharacterizing RF input drive power levels applied to the MZ modulator.The characterizing RF drive power levels applied to the MZ modulatorsare varied by varying the RF output power level of a 1 GHz RF signalfrom the signal generator 152.

To produce the modulator drive signals, the single-ended output of theRF sinusoidal signal source 152 is converted to a differential input byusing a 180° RF hybrid 154, which may be a 3-GHz hybrid. Thedifferential output of the 180° RF hybrid 154 is directed to thedifferential inputs of the RF modulator driver amplifiers 128, 130, 132,and 134 using a first and second 4-position RF switches 160, 170. Themodulator drive amplifiers 128, 130, 132, and 134 produce a single-endeddrive input to each child MZ modulator 104, 106, 108, and 110. In someembodiments, the RF input power provided by the modulator driveamplifier output 208 to the single-ended drive input to the MZ modulatoris determined by reading out the DC voltage of a RF peak detector 210 ineach of the particular driver amplifiers 128, 130, 132, and 134 in thetransmitter optical subassembly 102.

In some embodiments, the varying RF input power sweeps the MZ modulatordrive voltage from low voltage through the nominal V_(π) of the MZmodulator. The RF-modulated optical power resulting from the swept RFinput power is measured with the optical power meter 180. The MZmodulator drive voltage sweep is achieved by sweeping the power level ofthe signal generator 152 from low power to high power. The 180° RFhybrid 154 generates a differential output from the signal generator152, and the RF switches 160, 170 are controlled to select which MZmodulator 104, 106, 108, and 110 is driven by the swept signal. In someembodiments, the RF input power to the MZ modulator is determined byreading the value of the RF peak detector voltage as a readout from themodulator driver amplifier 128, 130, 132, and 134.

In some embodiments, the entire optical transmitter modulator structureis biased to a minimum transmission state before the RF power is sweptfor each modulator sequentially by changing the switch state of the two4-position RF switches 160, 170. Each MZ modulator is biased fornear-zero optical output power at the output 120. In some embodiments,the output power of the optical modulator in the minimum transmissionstate is less than −45 dBm. The biasing at a minimum transmission statehelps to ensure that there is no unmodulated optical power that wouldskew the measurement data. The measurement of the modulated (AC) opticalpower, that is at a frequency of 2 GHz in some embodiments, that is usedto determine IQ imbalance and XY power imbalance cannot include theunmodulated CW optical power (DC) because it will skew the measurementdata. Consequently, the modulators are biased at null transmission sothat unmodulated CW optical power (DC) shall not leak through to theoptical transmitter output 120. The electro-optic RF transfer functionsof each of the four respective MZ modulator 104, 106, 108, and 110 aremeasured successively by applying a 1 GHz RF sinusoidal signal ofvarying RF input power levels to each of the four respective MZmodulator's RF input at a given time, while maintaining the DC bias ofall MZ modulators biased at minimum transmission. In some embodiments,the sweeping of the RF input power to the MZ modulators comprisesvarying the RF output power of the 1 GHz sinusoidal signal sourcebetween −15 dBm and +6 dBm. The exact value of the swept RF output powerof the 1 GHz sinusoidal signal source may also depend upon the child MZmodulator's input RF Vπ voltage. As will be clear to those skilled inthe art, the MZ modulators may also be biased at other bias points whilenot departing from the apparatus and methods of characterization andcalibration of the present teaching. For example, the initial biascondition for the child MZ modulators may be a quadrature operatingpoint.

In some embodiments, the two 4-position RF switches 160, 170 areoperated to sequentially drive each child MZ modulator. As such, thefirst RF switch 160 is configured to connect its input to output port162 and the second RF switch 170 is configured to connect its input tooutput port 172. These output ports are connected to the positive andnegative inputs of the modulator drive amplifier 128 connected to childMZ modulator MZM-XI 104. In many methods, the modulator drive amplifier128 is only powered up during the measurement of the electro-optic RFtransfer functions and then powered down. Then the two 4-position RFswitches are reconfigured to connect their inputs to outputs 164, 174.The modulator drive amplifier 130 connected to MZM-XQ 106 is thenpowered up. Then the switch states are reconfigured to drive each of theother two modulator drive amplifiers 132, 134 connected to the remainingtwo child MZ modulators 108, 110 in the same manner. In various methodsaccording to the present teaching, the order of the switching betweenvarious modulator drive amplifiers applied to various child MZmodulators may be any order.

The output of the first step 302 of the method of the present teachingis a set of measured electro-optics RF transfer functions, which is theRF-modulated optical output power as a function of the RF input power,for each child MZ modulator in the transmitter optical subassembly 102at various optical wavelengths. In some embodiments, the wavelengthdependence of the electro-optic RF transfer function curves aredetermined by tuning the laser source to various wavelengths acrossC-band, and then generating a transfer function curve for eachwavelength. In some embodiments, the wavelengths represent eachwavelength on the ITU grid across the full C-band wavelength range. Forexample, the wavelengths can represent 90-100 channels at 50 GHz spacingin an amplifier's C-band. In various embodiments, different wavelengthranges and/or particular wavelength values are measured.

The second step 304 of the method 300 is to compute wavelength dependentV_(π) voltage for each MZ modulator using the electro-optic RF transferfunction data generated in the first step 302. The P1-dB compressionpoint is the input drive voltage to the modulator at which the opticaloutput power of the modulator drops by 1 dB from a linear dependence onthe MZ modulator drive voltage. The second step 304 of the method 300determines the operating points by curve fitting. For example, thesecond step 304 includes computing at least one of a linear fit, apolynomial fit, or an inverse cosine functional fit to the RF transferfunction data for each MZ modulator at a particular wavelength.

FIG. 4 illustrates a plot 400 of the simulated P1-dB compression pointdetermination used to compute Vπ voltage of a MZ modulator for oneembodiment of the method 300 according to the present teaching. Thesquares 402 on the plot 400 represent experimental data used for alinear fit 406 of the RF transfer function. The circles 404 representthe experimental data used for a polynomial fit 408. Also plotted arethe linear fit curve 406 and the polynomial fit curve 408. In somemethods, the P1-dB compression point is the MZ modulator drive voltagepoint at which the polynomial curve 408 of optical power falls below thelinear fit curve 406 of optical power by 1-dB. In some methods, theP1-dB compression point is the MZ modulator drive voltage point at whichan inverse cosine functional fit curve of the optical power falls belowthe linear fit curve of optical power by 1-dB.

The wavelength dependent P1-dB compression point computed for each MZmodulator in the second step 304 can be converted to a wavelengthdependent V_(π) for each modulator. The V_(π) voltage for each modulator104, 106, 108, and 110 is a scaled version of P1-dB voltage.Equivalently, the P1-dB voltage is the peak-to-peak voltage at 1-dBcompression point when the respective child MZ modulator is biased atthe minimum transmission. This voltage is scaled by a scalar of value1.3 to determine a V_(π) for each child MZ modulator. Thus, the outputof the second step 304 of the method 300 of the present teaching arewavelength dependent operating points of each MZ modulator in theoptical transmitter modulator, and include 1-dB compression pointsand/or V_(π) values.

In the third step 306 of the method 300, functional curve fit parametersare computed for the RF transfer function data of each MZ modulator atvarious wavelengths. The functional form of the curve fit may be alinear fit, a polynomial fit, an inverse cosine fit, or a curve fitfollowing any of numerous other functional relationships. In the case ofa linear fit, the functional parameters include the slope and theintercept of the linear fit. In this case, the third step 306 computes alinear curve fit to the measured wavelength dependent optical outputpower levels for each of the child MZ modulators versus MZ modulator RFdrive power levels.

In the fourth step 308 of the method, the functional parameters computedin the third step 306 for respective child MZ modulators are used toevaluate the wavelength dependent IQ power imbalance in dB for X-Pol.and Y-Pol. IQ modulators as well as X-Pol. and Y-Pol. IQ modulators'output power difference as XY polarization power difference at a desiredRF drive power level. The X-Pol. and Y-Pol. IQ modulators' output powerdifference as XY polarization power difference at a desired RF drivepower level is also known as transmitter polarization dependent loss(PDL). In methods according to the present teaching using a linearfunctional fit, the fourth step 308 of the method 300 uses the slopesand intercepts computed in the third step 306 for respective child MZmodulators to evaluate the wavelength dependent IQ power imbalance in dBand XY polarization power difference at a desired RF drive power level,or particular MZ modulator operating point.

One feature of the present teaching is the ability to determine theimpairment-induced power imbalances, and subsequent compensation setpoints which are described below, at select operating points of the MZmodulators in the optical modulator. By using a curve-fitting approachto determine functional parameters associated with each RF transfercurve for each child MZ modulator, the method of the present teachingcan quickly calculate the power imbalances at any of a variety ofmodulator operating points. The use of the curve fitting methoddescribed herein advantageously reduces the computational time requiredto determine the optical power at a particular RF drive power ascompared to known approaches that relies on look-up tables generated bythe transfer curve measurement data. In various methods according to thepresent teaching, different modulator operating points are used, forexample, when the transmitter operates with different modulationformats, such as either DP-QPSK or DP-16QAM modulation formats.

In some embodiments, the fourth step 308 determines wavelength dependentX-Polarization and Y-Polarization IQ-power imbalance for respective IQMZ modulators per polarization and a XY-power imbalance indual-polarization IQ MZ modulators at P1-dB compression point. In someembodiments, the power imbalances are determined at operating pointscomprising one or several RF input powers in a range of 10 dBm-13.5 dBm.Such a range is appropriate, for example, for a 16-QAM modulationscheme. In some methods according to the present teaching, the powerimbalances are determined at operating points comprising one or severalRF input powers that are in a range of 12 dBm-15 dBm, as appropriate fora QPSK modulation scheme.

FIG. 5 illustrates a plot 500 of the measured RF transfer functions forfour MZ modulators embedded into a wavelength tunable InP-based opticaltransmitter that modulates XI, XQ, YI, and YQ RF signals onto lightemitted from a tunable laser source operated at a particular wavelength.The plot 500 shows measured P1-dB compression points at an RF inputpower of +15 dBm of four MZ modulators. The data are presented for awavelength of 193.3 THz. The data show modulator response beforecompensation in RF domain and clearly shows the XY power imbalance, alsoknown as transmitter polarization dependent loss (PDL), of ˜2 dB.

In step five 310 of the method 300, the initial RF drive power levelsrequired to compensate IQ imbalance is computed for each parent MZmodulator representing modulation of each polarization, X-Pol. andY-Pol. In particular, for each of the two parent MZ modulators, the RFdrive power level required to compensate ½ of the wavelength dependentIQ power imbalance by under driving the first child MZ modulator whoseoptical output power is higher compared to the second child MZmodulators' optical output power is determined. The RF drive power levelrequired to compensate the remaining ½ of the wavelength dependent IQpower imbalance by over driving the second child MZ modulator in orderto match their optical output power levels is also determined. The overdrive and under drive are reversed in methods where first child MZmodulator has lower optical output power compared to the second child MZmodulators' optical output power in each polarization. The determined RFpower levels are referred to as initial RF drive power levels, andrepresent the value of RF power applied to each of the four child MZmodulators that will reduce or compensate for IQ power imbalance in eachparent X-Pol. and Y-Pol. IQ modulator.

In some methods according to the present teaching, the initial RF drivepower levels required for compensation are determined directly usingslopes and intercepts of the linear curve fits determined in step three306. In other methods, the initial RF drive power levels are determinedby comparing the parameters for fitted curves with other functionalrelationships, such as parabolic, inverse cosine, or other functionalrelationships of the RF transfer function data for the various MZmodulators. In these methods, the initial RF drive power levels aredetermined for various wavelengths based on the available data and curvefits for each child modulator.

In step six 312 of the method 300, the optical output powers of the twoparent MZ modulator modulators is determined when driving at therespective initial RF drive power levels for each child MZ modulator.Thus, step six 310 computes X-Pol. and Y-Pol. IQ modulators' opticaloutput powers at the initial RF drive power levels required tocompensate respective wavelength dependent IQ power imbalances for eachpolarization IQ modulators that was determined in step five 310.

Step six 312 of the method 300 also computes the wavelength dependent XYpower difference, which is the residual transmitter PDL that remaineduncompensated after compensating the wavelength dependent IQ powerimbalances of each of the IQ modulators in each polarization. Residualtransmitter PDL also indirectly contributes to the overall XY powerimbalance, or transmitter modulator PDL. The values of the opticaloutput powers at the initial RF drive power levels are determined byusing the functional curve fitted parameters for each child MZ modulatorderived in step three 306 of method 300.

In step seven 314 of method 300, the operating RF drive power levelsthat are required to compensate for XY power imbalance and to compensatefor IQ power imbalance of the optical output powers determined in stepsix 312 are determined. In some methods, step seven 314 computes theoperating RF drive power levels required to compensate ½ of thewavelength dependent residual XY polarization power imbalance by underdriving the pair of child MZ modulators embedded in first polarizationIQ modulator whose optical output power level is higher compared tosecond polarization IQ modulators' optical output power. The remaining ½of the XY power imbalance is compensated by over driving the pair ofchild MZ modulators embedded in the second polarization IQ modulator inorder to match their optical output power levels. The over driving andunder driving of respective pairs of child MZ modulators is reversed forthe case where the first polarization IQ modulator has lower opticaloutput power compared to second polarization IQ modulators' opticaloutput power.

The operating RF drive power levels are then determined by using thefunctional curve fitting parameters for each MZ modulator at multiplewavelengths that were derived in step three 306 of method 300. In somemethods using linear curve fitting, the operating RF drive power levelsare determined using slopes and intercept of the linear curve fittingthat were determined in step three 306. In other methods using inversecosine curve fitting in step three 306, the peak-to-peak RF drivevoltages of the respective child MZ modulators are computed so as tocompensate for the IQ power imbalance and for the XY power imbalance byover and/or under driving the respective child MZ modulator whose outputpower is higher compared to its counterpart by using the inverse of thecosine function that models the modulator's optical modulation loss fora Vπ voltage that is computed from the P1-dB compression point orcorresponding P1-dB voltage.

Step eight 316 of the method 300 determines the control set points forthe modulator drive amplifiers for each child MZ modulator based on theoperating RF drive power levels derived in step seven 314. In somemethods, the control set points are RF automatic gain control set pointsin peak-to-peak voltage derived from the operating RF drive powerlevels. The control set points are determined on a per wavelength basisfor each of the four child MZ modulators to compensate for thewavelength dependent IQ power imbalance in each polarization IQmodulator and to compensate for the XY power imbalance in the dualpolarization IQ modulator.

In step nine 318 of the method 300, the control set points are loadedinto the modulator drive amplifier controller. In some embodiments ofthe present teaching, the set points are provided to firmware thatcontrols the modulator drive amplifier. The outputs of the linear RFmodulator driver amplifiers are thus controlled in an automatic gaincontrol mode of operation to unequally drive each child MZ modulator sothat the InP-based optical transmitter generates a near-ideal DP-QPSKand DP-16QAM signal with less than ±0.1 dB of IQ-power imbalance perpolarization and XY-power imbalance over the full C-band.

In some embodiments, the linear RF modulator driver amplifiers areoperated in automatic gain control mode of operation to unequally drivethe respective RF inputs of the child MZ modulators in order tocompensate for the wavelength dependence of IQ power imbalance and forthe wavelength dependence of XY power imbalance in an InP based opticaltransmitter. In some methods, a constant output voltage set point isused for each modulator drive amplifier. In these methods, the outputdrive levels are independent of the input levels within an input dynamicrange of about 6 dB.

One feature of the methods and apparatus of the present teaching is thatthey have the ability to compensate for both optical polarizationdependence and for IQ power imbalance in InP-based optical transmittersthat provide tunable optical outputs in dual-polarization QPSK and QAMdata formats at high data rates. In addition, these methods andapparatus are effective at ITU wavelengths across the entire C-band ofcommonly used optical amplifiers.

FIG. 6A illustrates the optical transmitter modulator power imbalancesas a function of wavelength before compensation. Data are presented forITU frequencies across the C-band. Power imbalances near 1 dB areevident, and in all cases, the power imbalances exceed 0.2 dB in severalregions of the spectrum.

FIG. 6B illustrates the transmitter modulator power imbalances as afunction of wavelength after compensation using the method and apparatusof the present teaching. IQ power imbalances are near or below 0.2 dBacross the entire spectrum, and XY power imbalance is near or below 0.2dB across a majority of the spectral range. Comparing the data presentedin FIG. 6A to the data presented in FIG. 6B, it should be clear to oneskilled in the art that the compensation according to the presentteaching substantially reduces the IQ imbalance in X-Pol. and Y-Pol.modulation, and also substantially reduces the XY imbalance of thetransmitter modulator across the entire range of the C-band spectrum.

FIGS. 7A and 7B illustrate measured data from a DP-QPSK transmitterwithout compensation. More specifically, the measured data shown inFIGS. 7A and 7B illustrates the measured constellation of a 31.785-Gb/sDP-QPSK signal with no RF compensation. The data from the opticalmodulation analyzer shows a XY power imbalance of 1.8 dB.

FIGS. 8A and 8B illustrate measured data from a DP-QPSK transmitterusing a compensation method according to the present teaching. Morespecifically, the data shown in FIGS. 8A and 8B illustrates the measuredconstellation of a 31.785-Gb/s DP-QPSK signal with RF compensation. Theoptical modulation analyzer data show a XY power imbalance of less than0.2 dB and illustrates a known power transfer curve as a function of theratio of load resistance to source resistance.

One feature of the method for characterizing and compensating foroptical impairments in an optical transmitter of the present teaching isthat it is possible to reduce the test and compensation time by usingone of various interpolation techniques according to the presentteaching. Some embodiments of methods according to the present teachingmeasure and compensate IQ power imbalance and XY power imbalance, orpolarization dependent loss (PDL), per wavelength over the entire C-bandof an erbium-doped fiber amplifier comprising 96 DWDM wavelengthchannels on a 50 GHz frequency grid. A per-wavelength-based calibrationtest takes almost 12 hours to complete in the factory in a productionenvironment, which add significantly to the manufacturing costs andsignificantly decreases manufacturing capacity.

One aspect of the present teaching is the recognition that the patternof the measured IQ power imbalance and/or XY power imbalance over C-bandcan be empirically determined in the laboratory and used for variousinterpolation methods to reduce the time required for calibrationtesting. The empirically determined patterns of the measured IQ powerimbalance and/or XY power imbalance over C-band are used as a basis fora scheme in which IQ power imbalance and/or XY power imbalance is notmeasured on every wavelength, which can be a channel on an ITU grid. Forexample, in some embodiments, DWDM channels are measured to determine IQpower imbalance and/or XY power imbalance only after a predeterminednumber, such as every four, or every eight wavelengths. Thesemeasurements are then used to interpolate the RF AGC set points using apolynomial fit, such as a 5^(th) order polynomial fit, for the DWDMchannels that were not measured. One skilled in the art will appreciatethat numerous other interpolation methods known in the art can be used.This results in an accurate RF AGC set point for each channel in theDWDM channel spectrum, but with substantially reduced test andcalibration time.

Another aspect of the present teaching is that the optical transmitterscan generate optical signals with high spectral efficiency. In order toincrease the spectral efficiency, state-of-the-art digital signalprocessors (DSP) designed for coherent optical communication thatincorporate high-speed digital-to-analog converters within the DSP canbe used.

FIG. 9 illustrates a block diagram of an embodiment of an opticaltransmitter 900 of the present teaching that includes a transmit digitalsignal processor (DSP) module 936 comprising high-speeddigital-to-analog converters. The transmitter optical sub-assembly(TOSA) 902 is similar to the TOSA 102 of FIG. 1. The TOSA 902 includesfour child Mach-Zehnder modulators 904, 906, 908, 910 that form parentMZ modulators 912, 914. The first parent MZ modulator generates anX-polarized modulated optical beam, and the second parent MZ modulatorgenerates an Y-polarized modulated optical beam. The first and secondparent MZ modulators are configured to form a dual-polarization opticaltransmitter. In some embodiments, the transmitter generates a wavelengthtunable modulated optical signal. The wavelength tunable signal can tuneto various wavelength channels on the ITU grid. In some embodiments,there are more than two child MZ modulators in each parent MZ modulator.

The outputs of four modulator drive amplifiers 928, 930, 932, 934provide an RF signal to the electrical input of respective child MZmodulators 904, 906, 908, 910. The differential inputs to the modulatordrive amplifiers 928, 930, 932, 934 connect to the transmit DSP module936.

The optical input 916 is optically coupled to a tunable laser 918. Theoptical output 920 combines the output of parent MZM 912 and parent MZM914 after a polarization rotator 926. Parent MZM 912 includes a π/2phase shifter 922, and parent MZM 914 includes a π/2 phase shifter 924.There are also internal monitor photodetectors 938, 940 that have inputsthat are optically coupled to the output of respective parent MZs 912,914, that monitor the output power and provide that information to thecontrol system. Some embodiments include a third internal monitorphotodetector (not shown) that monitors the output 920 and providesfeedback information to the control system. The photodetectors areconnected to the DSP module that includes control processors.

The transmit DSP module 936 includes high-speed digital-to-analogconverters (DACs) 942, 944, 946, 948. These DACs 942, 944, 946, 948 eachprovide two complementary outputs, positive and negative, that are inputto the complementary inputs, positive (XIP, XQP, YIP, YQP) and negative(XIN, XQN, YIN, YQN) of respective modulator drive amplifiers 928, 930,932, 934. The child MZM 904 generates the x-polarized in-phase signal.The child MZM 906 generates the x-polarized quadrature signal. The childMZM 908 generates the y-polarized in-phase signal. The child MZM 910generates the y-polarized quadrature signal.

The digital inputs for the DACs 942, 944, 946, 948 are generated by theDSP in response to the control system processor (not shown explicitly inFIG. 9) that is part of the DSP. For example, the DACs 942, 944, 946,948 are capable of generating RF signals that, when used to drive thechild MZMs 904, 906, 908, 910, generate specific desired pulse shapesincluding, for example, bandwidth-efficient pulse shapes with variousdesired Nyquist roll-off factors. In some embodiments, the TOSA 902 isan InP TOSA, and the MZ modulators are InP modulators. The modulatordrive amplifiers 928, 930, 932, 934 are linear RF driver amplifiers andthe child MZMs 904, 906, 908, 910 are InP optical modulators.

Thus, DSPs incorporating high-speed digital-to-analog converters arecapable of performing digital signal processing operations, such asNyquist pulse shaping to reduce the signal bandwidth on the transmitside. The pulse roll-off factor in Nyquist pulse shaping veryeffectively controls the signal bandwidth. However, Nyquist shapedsignals have different peak-to-average power ratios for pulse roll-offfactors ranging from 0.01 to 1. The Nyquist signals having highpeak-to-average power ratios (PAPR) when amplified using RF driveramplifiers cause signal distortion in the RF driver due to clipping andoverdrive of the optical Mach-Zehnder modulators. In some embodiments ofthe present teaching, the RF AGC set points are scaled duringcalibration to account for particular PAPR values for a particularNyquist pulse roll-off setting. For example, root-raised cosine pulseswith a pulse roll-off factor of 0.2 having PAPR of 7.55 dB are used forgenerating 16QAM modulation format at 32- or 40-GBaud.

Referring to FIG. 1 and FIG. 9, the DSP module 936 may also beconfigured to perform the function of the various RF componentsdescribed in FIG. 1, thereby allowing the characterization andcompensation method to be carried out in a small package. Specifically,the DSP module 936 will perform some or all of the functions of theprocessor 182, signal generator 152, hybrid 154, and RF switches 160,170 that are described in connection with FIG. 1 so that they producethe described drive signals to the modulator drive amplifiers 928, 930,932, 934 that provide the characterization and compensation functions.In some embodiments, internal photodetectors 938, 940 and/or an internalphotodetector that monitors optical output power are used to provide thefunction provided by the optical power meter 180 described in connectionwith FIG. 1. These photodetectors are connected to the DSP, and theoutput of the photodetector is used to control the RF signals generatedby the DACs 942, 944, 946, 948. In some embodiments, an external powermeter is used to measure the optical power at output 920, and the outputof the power meter is connected to the DSP to provide the signals thatare used to control the DACs 942, 944, 946, 948.

In some embodiments, the DSP 936 is configured to generate anelectro-optic RF transfer function for each of the child MZ modulators904, 906, 908, 910, determine curve fitting parameters for each of theplurality of electro-optic RF transfer functions, determine operatingpoints of each of the child MZ modulators 904, 906, 908, 910 using thecurve fitting parameters, determine an IQ power imbalance at aparticular operating point for each of the first and the second parentMZ modulators 912, 914 using the curve fitting parameters for each ofthe plurality of electro-optic RF transfer functions, determine initialRF input drive power levels applied to each of the child MZ modulators904, 906, 908, 910 that compensate for the determined IQ power imbalancefor each of the first and the second parent MZ modulators 912, 914,determine XY power imbalance of the optical transmitter at thedetermined initial RF input drive power levels using the curve fittingparameters, and determine operating RF input drive power levels that atleast partially compensate for the first and second IQ power imbalancesand for the XY power imbalance of the optical transmitter.

FIG. 10A illustrates a graph 1000 of the peak-to-average power ratio forpulse amplitude modulation with four levels (PAM4) as a function of theNyquist pulse roll-off factor. The graph 1000 includes a plot 1002showing empirical data for a square-root raised cosine signal and a plot1004 of a polynomial fit to the empirical data. The graph 1000 alsoincludes a plot 1006 showing empirical data for a raised cosine signaland a plot 1004 of a polynomial fit to the empirical data. FIG. 10Billustrates a graph 1050 of the optical power for sixteen-levelquadrature amplitude modulation (16QAM) as a function of the Nyquistpulse roll-off factor. The graph shows a plot 1052 of data for asquare-root raised cosine signal and a plot 1054 of data for a raisedcosine signal.

Some embodiments of the methods of the present teaching use theassumption of a rectangular pulse shape and a mean RF AGC set point indetermining the RF AGC set point. Rectangular pulse shapes haverelatively low peak-to-average power ratios. In order to useNyquist-pulse-shaped signals having different pulse roll-off factors, wecompute the mean RF AGC set points lowered by considering thepeak-to-average ratio due to nQAM signals and Nyquist pulse shaping.This will reduce signal distortion in the RF driver and optimally driveoptical Mach-Zehnder modulators and consequently generate a near-idealoptical signal for transmission.

FIG. 11A illustrates a graph 1100 of the peak-to-average power ratiosfor nQAM signals generated by an embodiment of an optical transmitter ofthe present teaching. The graph 1100 includes a plot 1102 of data for asquare QAM format, showing the peak-to-average power ratio in dB as afunction of QAM level number as well as QAM index. FIG. 11A alsoillustrates a plot 1104 of data for a cross QAM format.

FIG. 11B illustrates a graph 1150 of the peak-to-average power ratio fordifferent pulse roll-off factors for raised cosine filter transferfunction with 50% Tx/Rx split Nyquist pulse shaping generated by anembodiment of an optical transmitter according to the present teaching.

EQUIVALENTS

While the Applicant's teaching is described in conjunction with variousembodiments, it is not intended that the Applicant's teaching be limitedto such embodiments. On the contrary, the Applicant's teaching encompassvarious alternatives, modifications, and equivalents, as will beappreciated by those of skill in the art, which may be made thereinwithout departing from the spirit and scope of the teaching.

What is claimed is:
 1. A method for characterizing and compensating foroptical impairments in an optical transmitter, the method comprising: a)operating an optical transmitter comprising a first parent Mach-Zehnder(MZ) modulator and a second parent MZ modulator, wherein each of thefirst and second parent MZ modulators comprises a plurality of child MZmodulators; b) biasing each of the plurality of child MZ modulators inthe first and second parent MZ modulators at respective initialoperating points; c) generating an electro-optic RF transfer functionfor each of the plurality of child MZ modulators by measuring aplurality of optical output powers of the optical transmitter whilesweeping characterizing RF input drive power levels applied to each ofthe plurality of child MZ modulators; d) determining curve fittingparameters for each of the plurality of electro-optic RF transferfunctions; e) determining operating points of each of the plurality ofchild MZ modulators using the curve fitting parameters; f) determiningan IQ power imbalance at a particular operating point for each of thefirst and the second parent MZ modulators using the curve fittingparameters for each of the plurality of electro-optic RF transferfunctions; g) determining initial RF input drive power levels applied toeach of the plurality of child MZ modulators that compensate for thedetermined IQ power imbalance for each of the first and the secondparent MZ modulators; h) determining XY power imbalance of the opticaltransmitter at the determined initial RF input drive power levels usingthe curve fitting parameters; i) determining operating RF input drivepower levels that at least partially compensate for the first and secondIQ power imbalances and for the XY power imbalance of the opticaltransmitter; and j) generating an optical signal comprising aNyquist-pulse-shape at an output of the optical transmitter.
 2. Themethod of characterizing and compensating of claim 1 wherein theplurality of child MZ modulators comprise InP MZ modulators.
 3. Themethod of characterizing and compensating of claim 1 wherein the opticaltransmitter operates over a range of wavelengths.
 4. The method ofcharacterizing and compensating of claim 3 wherein the operating RFinput drive power levels that at least partially compensate for thefirst and second IQ power imbalances and XY power imbalance of theoptical transmitter are determined over the range of wavelengths.
 5. Themethod of characterizing and compensating of claim 1 wherein the firstparent MZ modulator generates a modulated optical beam having a firstpolarization and the second parent MZ modulator generates a modulatedoptical beam having a second polarization.
 6. The method ofcharacterizing and compensating of claim 1 wherein the biasing theplurality of child MZ modulators at the initial operating pointcomprises biasing the plurality of child MZ modulators at a minimumtransmission level.
 7. The method of characterizing and compensating ofclaim 1 wherein the biasing the plurality of child MZ modulators at theinitial operating point comprises biasing the plurality of child MZmodulators at a bias that results in an optical output power that isless than −45 dBm.
 8. The method of characterizing and compensating ofclaim 1 wherein the biasing the plurality of child MZ modulators at theinitial operating point comprises biasing the plurality of child MZmodulators at a quadrature point.
 9. The method of characterizing andcompensating of claim 1 wherein the characterizing RF input drive powercomprises characterizing RF input drive power over a particular RFfrequency that is in a range of 500 MHz to 3 GHz.
 10. The method ofcharacterizing and compensating of claim 1 wherein the sweepingcharacterizing RF input drive power applied to of each of the pluralityof child MZ modulators comprises sweeping the characterizing RF inputdrive power of each of the plurality of child MZ modulatorssequentially.
 11. The method of characterizing and compensating of claim1 wherein the sweeping characterizing RF input drive power comprisessweeping characterizing RF input drive power of each of the plurality ofchild MZ modulators through a respective V_(π) of the child MZmodulators.
 12. The method of characterizing and compensating of claim 1wherein the sweeping characterizing RF input drive power comprisesvarying an output power of an external RF signal generator coupled to anRF input of each of the plurality of child MZ modulators in a range of−15 dBm to +6 dBm.
 13. The method of characterizing and compensating ofclaim 1 wherein the particular operating point used to determine the IQpower imbalance is Vπ.
 14. The method of characterizing and compensatingof claim 1 wherein the particular operating point used to determine theIQ power imbalance is between 10 dBm and 13.5 dBm.
 15. The method ofcharacterizing and compensating of claim 1 wherein the particularoperating point used to determine the IQ power imbalance is between 12dBm and 15 dBm.
 16. The method of characterizing and compensating ofclaim 1 wherein the generating the electro-optic RF transfer functionfor each of the plurality of child MZ modulators by measuring theplurality of optical output powers of the optical transmitter whilesweeping characterizing RF input drive power levels applied to each ofthe plurality of child MZ modulators comprises reading sweptcharacterizing RF input drive power from an RF peak detector.
 17. Themethod of characterizing and compensating of claim 16 wherein the RFpeak detector is calibrated by measuring the response of the RF peakdetector at three or more applied input voltages.
 18. The method ofcharacterizing and compensating of claim 1 wherein the determining curvefitting parameters for each of the plurality of electro-optic RFtransfer functions comprises performing a linear curve fit.
 19. Themethod of characterizing and compensating of claim 1 wherein thedetermining curve fitting parameters for each of the plurality ofelectro-optic RF transfer functions comprises performing a polynomialcurve fit.
 20. The method of characterizing and compensating of claim 1wherein the determining curve fitting parameters for each of theplurality of electro-optic RF transfer functions comprises performing aninverse cosine curve fit.
 21. The method of characterizing andcompensating of claim 1 further comprising determining voltage setpoints of modulator drive amplifiers that drive the plurality of childMZ modulators using the operating RF drive powers.
 22. The method ofcharacterizing and compensating of claim 21 further comprisingperforming automatic gain control using the voltage set points of themodulator drive amplifiers to compensate power imbalances.
 23. A methodof characterizing and compensating for optical impairments in anInP-based optical transmitter, the method comprising: a) operating anoptical transmitter comprising an X-Pol. IQ and a Y-Pol. IQ modulator,wherein each of the X-Pol. IQ and the Y-Pol. IQ modulators comprise afirst and second child MZ modulator, over a range of wavelengths; b)biasing each of the first and second child MZ modulators in each of theX-Pol. IQ and the Y-Pol. IQ modulators at respective initial operatingpoints; c) generating an electro-optic RF transfer function for each ofthe first and second child MZ modulators in each of the X-Pol. IQ andthe Y-Pol. IQ modulators by measuring a plurality of optical outputpowers of the optical transmitter while sweeping a characterizing RFinput drive power applied to each of the first and second child MZmodulators in each of the X-Pol. IQ and the Y-Pol. IQ modulators for atleast some wavelengths in the range of wavelengths; d) determining curvefitting parameters for each of the electro-optic RF transfer functions;e) determining an IQ power imbalance for each of the X-Pol. IQ and theY-Pol. IQ modulators using the curve fitting parameters; f) determininginitial RF input drive powers that when applied to each of the first andsecond child MZ modulators in each of the X-Pol. IQ and the Y-Pol. IQmodulators compensate for each of the X-Pol. IQ and the Y-Pol. IQmodulator's determined IQ power imbalance; g) determining XY powerimbalance of the optical transmitter at the determined initial RF inputdrive powers for each of the first and second child MZ modulators ineach of the X-Pol. IQ and the Y-Pol. IQ modulators using the curvefitting parameters; and h) determining operating RF input drive powersfor each of the first and second child MZ modulators in each of theX-Pol. IQ and the Y-Pol. IQ modulators that at least partiallycompensate for the first and second IQ power imbalances and for the XYpower imbalance of the optical transmitter over the range ofwavelengths.
 24. The method of characterizing and compensating of claim23 wherein the range of wavelengths comprises a range comprisingninety-six wavelengths on a 50-GHz frequency grid.
 25. The method ofcharacterizing and compensating of claim 23 wherein the range ofwavelengths comprises a range that falls within a C-band of anerbium-doped fiber amplifier.
 26. The method of characterizing andcompensating of claim 23 wherein the generating the electro-optic RFtransfer function for each of the first and second child MZ modulatorsin each of the X-Pol. IQ and the Y-Pol. IQ modulators comprisesgenerating the electro-optic RF transfer function for each of the firstand second child MZ modulators in each of the X-Pol. IQ and the Y-Pol.IQ modulators for only a predetermined number of wavelengths.
 27. Themethod of characterizing and compensating of claim 26 wherein thepredetermined number of wavelengths is either twelve or twenty-four. 28.The method of characterizing and compensating of claim 23 wherein thedetermining operating RF input drive powers for each of the first andsecond child MZ modulators in each of the X-Pol. IQ and the Y-Pol. IQmodulators that at least partially compensate for the IQ powerimbalances and XY power imbalance of the optical transmitter aredetermined over the range of wavelengths comprises performing apolynomial fit for wavelengths that are not measured.
 29. The method ofcharacterizing and compensating of claim 28 wherein the polynomial fitcomprises a fifth order polynomial fit.
 30. The method of characterizingand compensating of claim 23 wherein the biasing the first and secondchild MZ modulators of the X-Pol. IQ and the Y-Pol. IQ modulators at theinitial operating point comprises biasing the first and second child MZmodulators of the X-Pol. IQ and the Y-Pol. IQ modulators at a minimumtransmission level.
 31. The method of characterizing and compensating ofclaim 23 wherein the biasing the first and second child MZ modulators ofthe X-Pol. IQ and the Y-Pol. IQ modulators at the initial operatingpoint comprises biasing the first and second child MZ modulators of theX-Pol. IQ and the Y-Pol. IQ modulators at bias points that results in anoptical output power that is less than −45 dBm.
 32. The method ofcharacterizing and compensating of claim 23 wherein the biasing thefirst and second child MZ modulators of the X-Pol. IQ and the Y-Pol. IQmodulators at the initial operating point comprises biasing the firstand second child MZ modulators of the X-Pol. IQ and the Y-Pol. IQmodulators at a quadrature point.
 33. The method of characterizing andcompensating of claim 23 wherein the sweeping the characterizing RFinput drive power of each of the child MZ modulators comprises sweepingthe power of each child MZ modulator sequentially.
 34. The method ofcharacterizing and compensating of claim 23 wherein the sweeping thecharacterizing RF input drive power comprises sweeping thecharacterizing RF input drive power of each child MZ modulator throughtheir respective V_(π).
 35. The method of characterizing andcompensating of claim 23 wherein the measuring the plurality of opticaloutput powers of the optical transmitter as a function of the sweptcharacterizing RF input drive power of each of the child MZ modulatorscomprises reading input drive power from an RF peak detector.
 36. Themethod of characterizing and compensating of claim 35 wherein the RFpeak detector is calibrated by measuring the response of the RF peakdetector at three or more applied input voltages.
 37. The method ofcharacterizing and compensating of claim 23 wherein the determiningcurve fitting parameters for each of the electro-optic RF transferfunction of the first and second child MZ modulators of the X-Pol. IQand the Y-Pol. IQ modulators comprises performing a linear curve fit.38. The method of characterizing and compensating of claim 23 whereinthe determining curve fitting parameters for each of the electro-opticRF transfer function of the first and second child MZ modulators of theX-Pol. IQ and the Y-Pol. IQ modulators comprises performing a polynomialcurve fit.
 39. The method of characterizing and compensating of claim 23wherein the determining curve fitting parameters for each of theelectro-optic RF transfer function of the first and second child MZmodulators of the X-Pol. IQ and the Y-Pol. IQ modulators comprisesperforming an inverse cosine curve fit.
 40. The method of characterizingand compensating of claim 23 further comprising determining voltage setpoints of modulator drive amplifiers that drive the first and secondchild MZ modulators of the X-Pol. IQ and the Y-Pol. IQ modulators usingthe operating RF input drive powers.
 41. The method of characterizingand compensating of claim 40 further comprising performing automaticgain control using the voltage set points of the modulator driveamplifiers to compensate power imbalances.
 42. An optical transmitterthat characterizes and compensates for optical impairments, the opticaltransmitter comprising: a) a first parent Mach-Zehnder (MZ) modulatorcomprising a plurality of child MZ modulators, the first parent MZmodulator generating an X-polarized modulated optical beam; b) a secondparent Mach-Zehnder (MZ) modulator comprising a plurality of child MZmodulators, the second parent MZ modulator generating an Y-polarizedmodulated optical beam, the first and second parent MZ modulatorconfigured to form a dual-polarization optical transmitter thatgenerates wavelength tunable modulated optical signals; c) a pluralityof modulator drive amplifiers, each of the plurality of modulator driveamplifiers supplying modulation signals to respective child MZmodulators; d) a tunable laser source having an output that is opticallycoupled to an optical input of the dual-polarization opticaltransmitter; e) an optical photodetector having an input that isoptically coupled to an output of the dual-polarization opticaltransmitter, the optical photodetector detecting RF modulation initiatedby a signal generator and imposed by the child MZ modulators onto thewavelength tunable optical signals generated by the first and secondparent MZ modulator configured to form the dual-polarization opticaltransmitter; and f) a digital signal processor comprising a plurality ofdigital-to-analog converters each having a plurality of RF outputs thatare electrically connected to respective RF inputs of the plurality ofmodulator drive amplifiers that drive the child MZ modulators, thedigital signal processor being configured to cycle RF modulation signalsso that responses of each child MZ modulator to the RF modulationsignals can be characterized, the digital signal processor having aninput that is electrically connected to the optical photodetectors andconfigured to: i) generate an electro-optic RF transfer function foreach of the plurality of child MZ modulators; ii) determine curvefitting parameters for each of the plurality of electro-optic RFtransfer functions; iii) determine operating points of each of theplurality of child MZ modulators using the curve fitting parameters; iv)determine an IQ power imbalance at a particular operating point for eachof the first and the second parent MZ modulators using the curve fittingparameters for each of the plurality of electro-optic RF transferfunctions; v) determine initial RF input drive power levels applied toeach of the plurality of child MZ modulators that compensate for thedetermined IQ power imbalance for each of the first and the secondparent MZ modulators; vi) determine XY power imbalance of the opticaltransmitter at the determined initial RF input drive power levels usingthe curve fitting parameters; and vii) determine operating RF inputdrive power levels that at least partially compensate for the first andsecond IQ power imbalances and for the XY power imbalance of the opticaltransmitter.
 43. The optical transmitter of claim 42 wherein thedual-polarization optical transmitter comprises a dual-polarizationin-phase (I) and quadrature (Q) optical modulator and the tunable lasersource comprises a full C-band-wavelength thermally-tunable lasersource.
 44. The optical transmitter of claim 42 wherein the plurality ofmodulator drive amplifiers comprise differential inputs.
 45. The opticaltransmitter of claim 44 wherein the plurality of modulator driveamplifiers further comprises differential outputs.
 46. The opticaltransmitter of claim 44 wherein the plurality of modulator driveamplifiers further comprises single-ended outputs.
 47. The opticaltransmitter of claim 42 wherein the dual-polarization opticaltransmitter generates wavelength tunable modulated optical signals thatcomprise raised-cosine-shaped pulses with nQAM modulation.
 48. Theoptical transmitter of claim 42 wherein the dual-polarization opticaltransmitter generates wavelength tunable modulated optical signals thatcomprise square-root-raised-cosine-shaped pulses with nQAM modulation.